The invention relates to operation of well tools.
Oil field perforation and fracturing operations typically involve the use of explosives deployed downhole in gun systems. These explosively powered perforating systems, including bullet guns and shaped-charge perforators, when activated generate a geometrical pattern of perforations through casing and a cement sheath in the well and into the formation. Depending on the desired length and number of perforation zones, one or more gun strings of varying lengths are assembled and conveyed downhole on wireline, tubing, or coiled tubing.
A perforating apparatus typically comprises a gun section housing explosive and perforating devices and a firing head section housing a detonator. Typical explosive devices involved in perforating operations include shaped charges, detonating cords, detonators and powder charges. Detonators are classified as either electro-explosive devices (EEDs) or percussive devices. Percussive detonators are actuated by impact of a firing pin similar to the firing pin of a common rifle or handgun.
In penetrating guns armed with EEDs, inadvertent firing may occur at the surface because of (1) accidental application of electrical power to the gun, either as a pre-run check ("hot check") or in troubleshooting a misfired gun after recovery from the well; (2) entry of stray voltages from the wellsite into a faulty system; or (3) installation of a cable head onto an armed gun while power is applied to the cable.
Although safety procedures have been set up to prevent accidental firing, deviations from safe practices have resulted in injury to persons and damage to equipment. Typical safety procedures may include the following: (1) checking for voltage between a rig, casing, and cable armor before gun arming or disarming, and eliminating such voltage at the source; (2) installing grounding cables between the rig, casing and perforating unit; (3) avoiding the possibility of static potentials by not operating in electrical, dust or snow storms; (4) evacuating all nonessential personnel from the area; (5) eliminating radio-frequency hazards; and (6) disabling or isolating the perforating unit power system during gun arming and until the gun string is at least a certain distance into the well below ground level. Other safety procedures may also be implemented.
One major area of safety concern occurs at the surface prior to running the armed perforators downhole. After a tool string is assembled on the surface, a "hot check" or other surface test may be performed to verify the electrical integrity of the tool string and wireline. A hot check verification includes applying power to the tool string, confirming proper wiring connections and no leakage. The hot check test can be safely made only if the perforating gun is not connected to the wireline. Likewise, another important safety concern is ensuring that shooting power is not applied during the retrieval of a misfired gun.
Furthermore, it is also desirable to avoid detonation of the perforating gun until the gun string is placed in the desired location downhole and until all other desired operating parameters are met. Thus, a need exists for an improved safety system and technique to reduce accidents when handling perforating guns explosive devices, or other well tools.